This invention relates to a process for the preparation of .alpha.-L-aspartyl-L-phenylalanine methyl ester (.alpha.-APM). .alpha.-APM is well known for its usefulness as a sweetening agent.
Synthesis of .alpha.-APM, according to our previous invention of U.S. Pat. No. 3,933,781, proceeds in the following general sequence of reactions: ##STR1##
In the above equations X represents an amino protecting group.
As shown in equation (1) the starting reactants are an N-protected-L-aspartic anhydride and L-phenylalanine which are reacted to form N-protected-.alpha.-L-aspartyl-L-phenyl-alanine. The amino protecting group can be any of those known to persons skilled in the art as exemplified by formyl, acetyl, benzoyl, substituted and unsubstituted carbobenzoxy, t-butoxycarbonyl and the hydrohalide salt. Particularly preferred is N-formyl-L-aspartic anhydride.
The N-protected-.alpha.-L-aspartyl-L-phenylalanine can be separated from the N-protected-.alpha.-L-aspartyl-L-phenyl-alanine and treated to remove the protecting group to obtain .alpha.-L-aspartyl-L-phenylalanine as in equation (2). Our previous process contemplated the isolation of .alpha.-L-aspartyl-L-phenyl-alanine which was then esterified with methanol, as in equation (3), to form .alpha.-APM.
As described in U.S. Pat. No. 3,933,781, the esterification reaction was preferably "carried out with as little water present as possible". Such an esterification reaction was described for purposes of illustration as being carried out in methanol in the presence of hydrogen chloride. At that time we were of the opinion that the presence of any significant amount of water during esterification would tend to decrease the desired esterification by causing undesired deesterification reactions to occur.
A preferred method of recovering the .alpha.-APM, prepared by our procedure of U.S. Pat. No. 3,933,781, was to convert it to the HCl salt which was recovered as a solid and converted to .alpha.-APM.
Such a solid HCl salt of .alpha.-APM is also described in U.S. Pat. No. 3,798,207 which utilized it in a purification procedure for obtaining .alpha.-APM by separation from .beta.-APM and other undesired by-products. In both of the previous procedures, the HCl salt was formed as a means of recovering .alpha.-APM after it had been prepared.
It is the primary object of this invention to provide an improved process for the preparation of .alpha.-APM.
Further objects, aspects and advantages of this invention will be apparent from the description which follows.
According to the present invention, there is provided an improved process for preparing .alpha.-APM comprising contacting .alpha.-L-aspartyl-L-phenylalanine with a reaction medium comprising water, methanol and a hydrogen halide which is hydrogen chloride or hydrogen bromide to form a solid hydrogen halide salt of .alpha.-APM, separating the solid hydrogen halide salt and converting the separated salt of .alpha.-APM.
The .alpha.-L-aspartyl-L-phenylalanine can be obtained by treating the N-protected-.alpha.-L-aspartyl-L-phenylalanine to remove the protecting group (equation 2). Any method suitable for removing protecting groups from amines is appropriate. Examples of such methods are catalytic hydrogenation and treatment with mineral acids or bases. It is preferred to remove the protecting group, particularly the formyl group, by acid hydrolysis. This hydrolysis can be carried out in, for instance, a dilute aqueous hydrochloric acid solution. The conversion to .alpha.-L-aspartyl-L-phenylalanine is usually very high, i.e., on the order of 95% or higher based on the N-protected-.alpha.-L-aspartyl-L-phenylalanine so treated. Another medium for such treatment is an acetic acid-hydrochloric acid aqueous solution.
.alpha.-L-aspartyl-L-phenylalanine can then be recovered by precipitation and liquid/solid separation. Such precipitation can, for instance, be produced by pH adjustment when the protecting group has been removed in an acid solution.
It is also possible with the process of the present invention to use the N-protected-.alpha.-L-aspartyl-L-phenylalanine to form the .alpha.-L-aspartyl-L-phenylalanine in situ in the reaction medium or to form the .alpha.-L-aspartyl-L-phenylalanine in a reaction medium without the need for isolation. A particularly preferred N-protected-.alpha.-L-aspartyl-L-phenylalanine useful in this latter manner is N-formyl-.alpha.-L-aspartyl-L-phenylalanine.
The amount of hydrogen halide useful in the reaction medium is from about 0.1 mole to about 0.80 mole per 100 grams of reaction medium. A particularly useful amount of hydrogen halide is from about 0.3 mole to about 0.7 mole per 100 grams of reaction medium. The amount of methanol useful in the reaction medium is from about 0.1 to about 1.1 moles per 100 grams of reaction medium. A particularly useful amount of methanol is from about 0.4 to about 0.6 mole per 100 grams of reaction medium.
It will be recognized by those skilled in the art that other materials may be included in the reaction medium.
The hydrogen halide present in the reaction medium must be present in an amount of from at least about 1.0 to about 20.0 moles of hydrogen halide per mole of .alpha.-L-aspartyl-L-phenylalanine being contacted. A particularly preferred amount is from about 1.15 to about 10.0 moles per mole of .alpha.-L-aspartyl-L-phenylalanine. Hydrogen chloride is the preferred hydrogen halide.
Those persons skilled in the art will recognize that the reaction medium must also contain at least about 1.0 mole of methanol per mole of .alpha.-L-aspartyl-L-phenylalanine and higher levels can also be utilized.
It should also be understood that the concentrations and amounts of materials used in the reaction medium and .alpha.-L-aspartyl-L-phenylalanine cannot practically be undertaken if undesirably excessive mixing problems are encountered.
When the .alpha.-L-aspartyl-L-phenylalanine is to be formed in situ, it has been found to be advantageous to add a lower level of hydrogen halide followed by a heating of the reaction up to about 65.degree. C. and cooling. This causes the hydrolysis of the N-protected-.alpha.-L-aspartyl-L-phenylalanine to .alpha.-L-aspartyl-L-phenylalanine. Subsequent to such heating, further aqueous hydrogen halide can be added to the reaction mass to provide a reaction medium as described above leading to the formation of the solid hydrogen halide salt of .alpha.-APM.
The temperatures utilized should be up to about the boiling point of the reaction mass. From about 5.degree. to about 50.degree. C. is preferred, particularly from about 20.degree. to about 40.degree. C. Although most preferred temperature is near ambient temperature, it should be noted that higher temperatures tend to increase the rate of formation of .alpha.-APM but have the disadvantages of causing decomposition reactions and increasing the solubility of the hydrogen halide salts of .alpha.-APM. On the other hand, lower temperatures tend to decrease the rate of formation of .alpha.-APM, inhibit decomposition reactions and give higher levels of solid hydrogen halide salts of .alpha.-APM. One skilled in the art will recognize the need to balance those considerations to achieve the most economical temperature for the concentrations involved.
Inherent in the reaction taking place in the process of this invention is the formation of the following by-products: ##STR2## (hereinafter referred to as the "diester") and ##STR3## (hereinafter referred to as the "aspartyl ester").
In addition to these two by-products, the reaction mass may also contain unesterified .alpha.-L-aspartyl-L-phenylalanine. The reactions leading to the desired product and by-products are all equilibrium reactions. In the procedure of our U.S. Pat. No. 3,933,781 the isolated yields of .alpha.-APM obtained were generally from about 25 to 30% based on .alpha.-L-aspartyl-L-phenylalanine.
It has now been discovered that the process of the present invention provides a large isolated yield of .alpha.-APM. For instance, at about room temperature, as much as about 55 to 60 percent .alpha.-APM yield, based on .alpha.-L-aspartyl-L-phenylalanine, can be obtained. This is particularly surprising in view of the isolated yields obtainable in our previous procedures.
The solid hydrogen halide salt of .alpha.-APM can be recovered by solid/liquid separation procedures. Essentially all of the other compounds remain in the mother liquor and can be hydrolyzed, recovered and/or recycled to the previous reactions. The separated salt can then be converted to substantially pure .alpha.-APM for instance, as shown in U.S. Pat. Nos. 3,798,207 and 3,933,781.